The biggest problem with the use of multiple drivers is their phase interaction. When two pistons are playing out of phase, you get comb filtering. Even worse, the comb filtering changes if you move even an inch, and it destroys your soundstage.
That's the bad news.
The good news is that our perception of the soundstage isn't particularly sensitive to frequency response below 1khz. (We ARE sensitive to path length distances, which is why many people are bothered by putting speakers in the door of their car.)
If you have some time to kill, fire up the stereo in your car. Now add a tweeter, one on each side.
The soundstage will collapse.
Now remove the extra tweeters, and add a midrange, located the same distance as your first set of mids. It doesn't even have to be near the primary set of mids, in fact it could be two feet below them.
You'll notice the soundstage is intact, but the system has a pleasant glow of ambience.
The reason that you can add mids easily is that the wavelengths are very long, so comb filtering isn't much of an issue below 1khz.
A 500hz sound wave is two and a half feet long.
